Merge branch 'docs-redis-requirements' into 'master'

Remove Redis 2/3 support from requirements document

See merge request gitlab-org/gitlab!39405

README.md

@@ -81,7 +81,7 @@ GitLab is a Ruby on Rails application that runs on the following software:
 - Ubuntu/Debian/CentOS/RHEL/OpenSUSE
 - Ruby (MRI) 2.6.5
 - Git 2.8.4+
-- Redis 2.8+
+- Redis 4.0+
 - PostgreSQL 11+
 
 For more information please see the [architecture](https://docs.gitlab.com/ee/development/architecture.html) and [requirements](https://docs.gitlab.com/ee/install/requirements.html) documentation.